Various causes of unwanted EMI have been noted and attempts have been made to eliminate such disturbances. U.S. Pat. No. 4,453,526 and its companion, U.S. Pat. No. 4,527,535 both recognize that magnetic fluxes leak from ignition coils and that this leakage can cause erroneous operation or malfunction of equipment such as integrated ignition coils and amplifier combinations.
U.S. Pat. No. 4,380,225 seeks to protect the ignition system of internal combustion engines against electrical and magnetic interference. In this patent, the solution includes the use of light guide "pipe" photoconductors which are in some instances included in the cable harness of the vehicle.
U.S. Pat. No. 4,463,224 relates to the suppression of electromagnetic waves radiating from the discharge between a distributing electrode and side terminal electrode. The solution for this patent includes the use of silicon carbide ceramics and other electromagnetic wave absorbing material. Primarily, the material is mixed appropriately with the distributor cap or applied thereon.
Finally, U.S. Pat. No. 4,186,712 is specifically drawn to the elmination of radio frequency interference from gasoline engines which have capacitor discharge ignition systems. This solution disclosed in this patent includes the use of special spark plugs and a low-resistance, low-inductance suppression element.
All of these patents recognize that EMI may be eliminated, at least in part, when the source of EMI is shielded or when the interference is absorbed into specifically provided elements. Shielding of wires which transmit high-voltage current is, of course, one method for eliminating EMI from a device. Conductive shields such as braided wire may be used to surround wires and effectively reduce EMI, as long as the braided wire shields themselves are properly grounded. Proper grounding of any type is useful in reducing EMI. Even where proper shielding is used, if grounding is not adequate, the EMI will escape and potentially disrupt suitability of the operation of the device.
In specific devices which are portable, such as the above-described gasoline engine powered, portable fire pumps, shielding the spark plug and the ignition wires has been found to reduce somewhat the amount of EMI present during operation. Nevertheless, such steps of shielding the spark plug and the ignition wires is not adequate to reduce the EMI to a level which would permit the use of electronics. Heretofore, it has only been possible to effectively reduce EMI from portable spark ignition-powered pumps when low energy ignition systems are employed. When high energy ignition systems are used with higher voltages than have been employed in the past for pumps of this type, the resultant electric fields are, therefore, also more intense. Ignition systems of over 40,000 volts with extremely rapid rise time are contemplated. Shielding is far more difficult, and has not been possible using the individual methods known previously.